The solution conformations of two tachykinins, substance P (Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2), physalaemin (pGln-Ala-Asp-Pro-Asn-Lys-Phe-Tyr-Gly-Leu-Met-NH2), senktide(Asp-Phe-Phe-Gly-Leu-Met-NH2) and septide (Asp-Phe-Phe-Pro- Leu-Met-NH2) have been studied here with a combination of NMR 2-dimensional NOE's and semi-empirical energy calculations. The number of possible conformations of these small peptides is extremely large, and requires a large computational effort to locate the low energy conformations of these peptides. Energy calculations are carried out for fragments to find low energy conformations by energy refinement. Then, these are used as possible starting conformations for the whole molecule. Thus, the starting conformations of the whole molecule are chosen by combining the low energy conformations of each fragment. Following this, the total conformational energies are calculated and energy refinements carried out. Conformations are then selected that are consistent with the NMR 2-dimensional NOE data. The receptor selectivity of these peptides appears to be related to their conformation. Results suggest these bindings might be enhanced by rigidifying the peptides in an appropriate way.